1. Field of the Invention
This invention relates to an acousto-optic tunable filter.
2. Discussion of Related Art
As an optical signal traverses an optical network, the signal is subject to losses and nonlinear effects that result in signal attenuation and distortion. Amplifiers, such as erbium-doped fiber amplifiers (“EDFA's”), are typically placed approximately every 80 kilometers along an optical fiber to boost signal strength. However, such amplifiers impose their own distortions on the signal power spectral distribution (as a function of wavelength). One of the major distortions is caused by the non-uniform gain profile (as a function of wavelength) of the amplifiers, which imposes a non-uniform spectral distribution on the amplified signals. It is especially important in wavelength division multiplexed (“WDM”) networks to maintain a uniform spectral distribution across all channels.
Static filters are often used to attenuate the signal power as a function of wavelength to achieve a substantially uniform power distribution. Static filters, however, cannot adapt to dynamically changing conditions such as amplifier aging, temperature variations, channel add/drop, fiber loss and other changes in components along the transmission line. Moreover, the required filter shape is dependent upon system configuration, e.g., the spacing between amplifiers. Static filter characteristics cannot be modified to compensate for these changes without replacing the filter itself.
To overcome these problems, it is known in the art to employ dynamic wavelength tunable filters to flatten or equalize the signal spectrum, as well as to obtain any desired spectral shape. One such filter is an all-fiber acousto-optic tunable filter (“AOTF”) described in U.S. Pat. No. 6,233,379, entitled “Acousto-optic filter, ” which is assigned to the assignee of the present invention and incorporated by reference herein. As described in the patent, the all-fiber AOTF is a multiple-notch filter, with a transfer function characterized by notch depth and center frequency (or wavelength).
The ATOF has a piezoelectric transducer that vibrates a conical wave propagation member. A tip of the conical wave propagation member vibrates an optical fiber. Transverse flexural waves are created in the optical fiber that filter certain wavelengths of light from a core into a cladding layer of the optical fiber.
One problem with the AOTF is that the wave is reflected back to the conical wave propagation member. Such a reflection interferes with the wave traveling from the conical wave propagation member, resulting in a modification microbending of the optical fiber. The modified microbending, in turn, creates modifications in wavelengths and/or magnitude of light that couples from the core into the cladding, and results in undesirable filtering characteristics.